1. Field of the Invention
The present invention relates to a fabrication method of a plastic-packaged semiconductor device and more particularly, to a fabrication method of a semiconductor device equipped with a semiconductor element chip such as an Integrated Circuit (IC) chip and a plastic package for protecting the chip, in which the package has a cavity near bonding pads or connecting terminals of the chip.
2. Description of the Prior Art
In general, microwave semiconductor elements, which are typically formed as IC chips, are designed to operate in the microwave band covering the high frequency region from approximately 1 GHz to 1000 GHz.
It has been known that the electrical characteristics of the microwave semiconductor element tend to degrade due to the parasitic capacitance caused by a dielectric (e.g., synthetic resin or ionic impurity) existing near the bonding pads or connecting terminals of the element or IC chip. Therefore, the microwave semiconductor element chip needs to be packaged in such a way that a hollow cavity is formed in the vicinity of the bonding pads inside the plastic package, thereby removing selectively the packaging plastic material from the vicinity of the bonding pads.
To cope with this need, a plastic package structure as shown in FIGS. 1 and 2 have been usually used.
As shown in FIGS. 1 and 2, a plastic-packaged semiconductor device 101 with the conventional package structure is comprised of a supporting member 102 made of a synthetic resin or plastic. This member 102 has an approximately cylindrical shape whose top face is depressed. The member 102 supports a die pad or land 103 and four lead fingers 104. The die pad 103 and the lead fingers 104 are fixed by the member 102 to have a specific positional relationship. Each of the die pad 103 and the lead fingers 104 is made of conductive metal. The member 102 is joined and bonded at a joining/sealing portion 109 to a cylindrical cap 107 having an approximately cylindrical shape whose bottom face is depressed. The cap 107 is made of a synthetic resin or plastic.
The cap 107 constitutes a plastic package of the conventional semiconductor device 101. The depression at the top of the supporting member 102 and the depression at the bottom of the cap 107 are coupled together to form a hollow cavity 108 within the cap 107 (i.e., the plastic package).
The joining and sealing of the cap 107 to the supporting member 102 are carried out by the use of a thermosetting epoxy resin, a thermoplastic resin, or the like. Each of the supporting member 102 and the cap 107 is typically made of a thermosetting epoxy resin.
A microwave semiconductor element chip (i.e., a microwave IC chip) 105 is fixed onto the surface of the die pad 103 using a suitable adhesive material. The bonding pads (not shown) of the chip 105 formed at its surface are electrically connected to the inner ends of the corresponding lead fingers 104 through bonding wires 106. The microwave IC chip 105 and the bonding wires 106 are located in the cavity 108 of the cap 107 serving as the plastic package.
As seen from FIG. 1, two ones of the lead fingers 104 are arranged along a diameter of the supporting member 102, and the remaining two ones of the lead fingers 104 are arranged along another diameter of the member 102 perpendicular thereto. Inner portions of the four lead fingers 104, which are located inside the member 102, constitute inner leads. Outer portions of the four lead fingers 104, which are located outside the member 102, constitute outer leads.
The outer leads protruding from the supporting member 102 are produced by cutting and separating them from a lead frame (not shown) at end portions 104a, and by bending them at bending portions 104b and 104c to have a shape shown in FIGS. 1 and 2.
The conventional plastic-packaged semiconductor device 101 shown in FIGS. 1 and 2 is fabricated in the following way.
First, by a known injection molding method or the like, a thermosetting epoxy resin is molded to have an approximately cylindrical shape at the specific location of the lead frame, thereby forming the supporting member 102 that supports the die pad 103 and the lead fingers 104 of the lead frame in the manner shown in FIGS. 1 and 2.
Next, without separating the die pad 103 and the lead fingers 104 from the lead frame, the microwave IC chip 105 is fixed onto the surface of the die pad 103 using a specific adhesive material. Then, the bonding pads of the chip 103 and the inner ends of the lead fingers 104 are respectively bonded together through the bonding wires 106, thereby electrically connecting the bonding pads of the chip 103 to the respective lead fingers 104. Thereafter, the cap 107 is mechanically connected to the supporting member 102 at the joining/sealing portion 109 under heat, thereby sealing the IC chip 105 from its outside.
Finally, the lead fingers 104 are cut at the positions 104a to be separated from the lead frame and then, the outer leads are bent at the positions 104b and 104c to have the shape shown in FIGS. 1 and 2. Thus, the conventional semiconductor device 101 with the package structure shown in FIGS. 1 and 2 is fabricated.
Other known fabrication methods of the semiconductor devices or high-frequency circuit elements of this sort will be explained below.
The Japanese Examined Patent Publication No. 45-22384 published in July 1970 discloses a fabrication method of a ceramic filter of the energy confinement type. In this method, after an energy-confinement type resonator is formed on a piezoelectric ceramic plate, a film is formed on the electrodes of the resonator with a material which is solid or semisolid at room temperature (15.degree. C.) and which is able to be melted under heat. Next, an insulating resin layer is formed to cover the solid or semisolid film by a dipping or molding method. During or after the formation of the insulating resin layer, the solid or semisolid film is melted due to heat and is absorbed into the insulating resin layer, thereby forming a cavity or gap at a location over the electrodes of the resonator within the insulating resin layer serving as a plastic package.
As the material which is solid or semisolid at room temperature and which is able to be melted under heat, the Japanese Examined Patent Publication No. 45-22384 discloses a paraffin or wax.
The Japanese Non-Examined Patent Publication No. 61-166138 published in July 1986 discloses a fabrication method of a semiconductor device, in which a semiconductor element chip is encapsulated by the use of a liquid resin along with an expanding or foaming agent such as dry ice and then, the resin is cured due to application of heat. After the expanding or foaming agent such as dry ice is placed on the surface of the chip, the chip and the agent are molded by the resin using a molding die. Due to the heat applied during the molding process, the resin is cured and at the same time, the agent is vaporized to be expanded, thereby forming a cavity of the package near the surface of the chip.
Light or vibration may be applied for curing the liquid resin instead of heat.
The Japanese Non-Examined Patent Publication No. 64-71138 published in March 1969 discloses a fabrication method of a plastic-packaged semiconductor device. In this method, after a surface-wave moving element is located between a transmitting electrode and a receiving electrode arranged apart from one another on the surface of an insulating substrate, a molten wax is coated to cover the whole surface of the substrate. Next, a porous resin film is formed to cover the film-shaped solid wax thus formed. The film-shaped solid wax and the porous resin film are reheated under a reduced pressure so that the solid wax is melted again and the melted wax is vaporized through the porous resin film, thereby forming a cavity between the element and porous resin film. Subsequently, the porous resin film is covered with a packaging or sealing plastic by a transfer molding method.
The Japanese Non-Examined Patent Publication No. 6-283619 published in October 1994 discloses a fabrication method of a semiconductor device. In this method, after a composite element including at least one elastic surface-wave element is provided on the surface of a substrate, a cavity-forming material is coated to cover the at least one elastic surface-wave element. Then, the surface of the substrate is covered with a porous elastic surface-wave absorption material. Subsequently, the cavity-forming material is heated to be vaporized, thereby forming a cavity in an encapsulating plastic package.
As the cavity-forming material, the Japanese Non-Examined Patent Publication No. 6-283619 discloses poly .alpha.-methyl styrene with a low ceiling temperature at which this material is completely vaporized, polyisobutylene or polymethacrylonitrile with a low decomposition temperature, or a mixture of waxes.
The above-described conventional semiconductor device 101 shown in FIGS. 1 and 2 has the following problems.
A first problem is that the fabrication cost is high because the cap 107 with the specific shape is necessary to be formed in advance.
This problem becomes remarkable if the miniaturization of the plastic package progresses further, which is because the positioning of the cap 107 needs to be precisely controlled using a special control system in the joining or bonding process of the cap 107 to the supporting member 102.
A second problem is that the reliability of the semiconductor device 101 is degraded.
Since a thermoplastic or thermosetting resin is used as the adhesive material for joining the cap 107 to the supporting member 102, heat needs to be applied during the joining process of the cap 107. Therefore, an air or gas confined in the cavity 108 of the package is expanded during the joining process.
On the other hand, the expanded air or gas in the cavity 108 gradually shrinks according to the lowering of the ambient temperature. As a result, a pressure difference is generated between the air or gas confined in the cavity 108 and the atmospheric air existing outside the package.
This pressure difference tends to cause a blowing-out phenomenon of the confined air or gas in the cavity 108 toward the outside or a sucking-in phenomenon of the atmospheric air toward the cavity 108, thereby forming penetrating pores in the adhesive material layer formed at the joining/sealing portion 109. The penetrating pores thus formed allow a soldering flux used in the mounting process of the semiconductor device 101 onto a printed wiring board and/or the atmospheric air existing in an operating environment of the device 101 to enter the inside of the plastic package, which degrades the reliability of the device 101.
A third problem is that moisture existing in an operating environment of the device 101 is likely to enter the inside of the plastic package through the cap 107 itself if the cap 107 becomes thinner according to the progressing device miniaturization.
With the above-described conventional fabrication methods disclosed in the Japanese Examined Patent Publication No. 45-22384, and the Japanese Non-Examined Patent Publication Nos. 61-166138, 64-71138, and 6-283619, the above-described first to third problems do not occur because any cap as shown in FIGS. 1 and 2 is not used. However, these conventional methods have the following problems.
With the conventional fabrication method disclosed in the Japanese Examined Patent Publication No. 45-22384, to form the cavity in the package, the film made of paraffin or wax is melted due to heat and then, the paraffin or wax film thus melted is removed by causing the melted film to be absorbed into the insulating resin film.
However, the melted paraffin or wax film is difficult to be removed completely and as a result, some residue of the paraffin or wax tends to be left on the surface of the resonator near the electrodes. This is because the resonator has small surface irregularity in the vicinity of the electrodes. Especially, the residue is readily left in the inter-electrode surface recesses of the resonator.
Since it has been known that the paraffin or wax residue damages or degrades the high-frequency characteristics of the resonator, the conventional method disclosed in the Japanese Examined Patent Publication No. 45-22384 has not been actually used in popular.
With the conventional fabrication method disclosed in the Japanese Non-Examined Patent Publication No 61-166138, dry ice is clearly disclosed as the expanding or foaming agent However, since dry ice vaporizes at an extremely high rate at room temperature, the amount and the temperature of the expanding or foaming agent are difficult to be controlled. As a consequence, it is extremely difficult to stably form the hollow cavity with a desired shape and a desired size at a desired position of the plastic package.
Additionally, since dry ice is solid at room temperature and has no wettability to the surface of the IC chip, the dry ice is likely to shift from its desired position during the molding process of the plastic package. This point increases the difficulty in formation of the cavity.
No other example than dry ice is disclosed in the Japanese Non-Examined Patent Publication No. 61-166138 as the expanding or foaming agent.
With the conventional fabrication method disclosed in the Japanese Non-Examined Patent Publication No. 64-71138, after the melted wax is coated and cooled to thereby form the wax layer, the porous resin film is formed to cover the wax layer. Thereafter, the wax layer is reheated under the reduced pressure to be melted. The melted wax layer is then vaporized through the porous resin film, thereby forming the cavity between the element and the porous resin film.
Therefore, it is essential for the resin film to be porous and accordingly, the packaging plastic is essentially formed to cover the porous resin film. If the packaging plastic is not used, the "moisture resistance" property of the plastic-packaged semiconductor device will degrade.
As explained above, with the conventional fabrication method disclosed in the Japanese Non-Examined Patent Publication No. 64-71138, both of the porous resin film and the packaging plastic need to be stacked in addition to the wax film. Therefore, the configuration of the plastic package becomes complicated.
With the conventional fabrication method disclosed in the Japanese Non-Examined Patent Publication No. 6-283619, after the cavity-forming material is coated to cover the at least one elastic surface-wave element, the whole surface of the substrate is covered with the porous elastic surface-wave absorbing material. Subsequently, the cavity is formed in the inside of the plastic package by heating and vaporizing the cavity-forming material.
Therefore, it is necessary to use a material with a low ceiling temperature or decomposition temperature as the cavity-forming material. This means that only limited materials can be used to form the plastic package.